| .. _regex-howto: |
| |
| **************************** |
| Regular Expression HOWTO |
| **************************** |
| |
| :Author: A.M. Kuchling <amk@amk.ca> |
| |
| .. TODO: |
| Document lookbehind assertions |
| Better way of displaying a RE, a string, and what it matches |
| Mention optional argument to match.groups() |
| Unicode (at least a reference) |
| |
| |
| .. topic:: Abstract |
| |
| This document is an introductory tutorial to using regular expressions in Python |
| with the :mod:`re` module. It provides a gentler introduction than the |
| corresponding section in the Library Reference. |
| |
| |
| Introduction |
| ============ |
| |
| The :mod:`re` module was added in Python 1.5, and provides Perl-style regular |
| expression patterns. Earlier versions of Python came with the :mod:`regex` |
| module, which provided Emacs-style patterns. The :mod:`regex` module was |
| removed completely in Python 2.5. |
| |
| Regular expressions (called REs, or regexes, or regex patterns) are essentially |
| a tiny, highly specialized programming language embedded inside Python and made |
| available through the :mod:`re` module. Using this little language, you specify |
| the rules for the set of possible strings that you want to match; this set might |
| contain English sentences, or e-mail addresses, or TeX commands, or anything you |
| like. You can then ask questions such as "Does this string match the pattern?", |
| or "Is there a match for the pattern anywhere in this string?". You can also |
| use REs to modify a string or to split it apart in various ways. |
| |
| Regular expression patterns are compiled into a series of bytecodes which are |
| then executed by a matching engine written in C. For advanced use, it may be |
| necessary to pay careful attention to how the engine will execute a given RE, |
| and write the RE in a certain way in order to produce bytecode that runs faster. |
| Optimization isn't covered in this document, because it requires that you have a |
| good understanding of the matching engine's internals. |
| |
| The regular expression language is relatively small and restricted, so not all |
| possible string processing tasks can be done using regular expressions. There |
| are also tasks that *can* be done with regular expressions, but the expressions |
| turn out to be very complicated. In these cases, you may be better off writing |
| Python code to do the processing; while Python code will be slower than an |
| elaborate regular expression, it will also probably be more understandable. |
| |
| |
| Simple Patterns |
| =============== |
| |
| We'll start by learning about the simplest possible regular expressions. Since |
| regular expressions are used to operate on strings, we'll begin with the most |
| common task: matching characters. |
| |
| For a detailed explanation of the computer science underlying regular |
| expressions (deterministic and non-deterministic finite automata), you can refer |
| to almost any textbook on writing compilers. |
| |
| |
| Matching Characters |
| ------------------- |
| |
| Most letters and characters will simply match themselves. For example, the |
| regular expression ``test`` will match the string ``test`` exactly. (You can |
| enable a case-insensitive mode that would let this RE match ``Test`` or ``TEST`` |
| as well; more about this later.) |
| |
| There are exceptions to this rule; some characters are special |
| :dfn:`metacharacters`, and don't match themselves. Instead, they signal that |
| some out-of-the-ordinary thing should be matched, or they affect other portions |
| of the RE by repeating them or changing their meaning. Much of this document is |
| devoted to discussing various metacharacters and what they do. |
| |
| Here's a complete list of the metacharacters; their meanings will be discussed |
| in the rest of this HOWTO. :: |
| |
| . ^ $ * + ? { [ ] \ | ( ) |
| |
| The first metacharacters we'll look at are ``[`` and ``]``. They're used for |
| specifying a character class, which is a set of characters that you wish to |
| match. Characters can be listed individually, or a range of characters can be |
| indicated by giving two characters and separating them by a ``'-'``. For |
| example, ``[abc]`` will match any of the characters ``a``, ``b``, or ``c``; this |
| is the same as ``[a-c]``, which uses a range to express the same set of |
| characters. If you wanted to match only lowercase letters, your RE would be |
| ``[a-z]``. |
| |
| Metacharacters are not active inside classes. For example, ``[akm$]`` will |
| match any of the characters ``'a'``, ``'k'``, ``'m'``, or ``'$'``; ``'$'`` is |
| usually a metacharacter, but inside a character class it's stripped of its |
| special nature. |
| |
| You can match the characters not listed within the class by :dfn:`complementing` |
| the set. This is indicated by including a ``'^'`` as the first character of the |
| class; ``'^'`` outside a character class will simply match the ``'^'`` |
| character. For example, ``[^5]`` will match any character except ``'5'``. |
| |
| Perhaps the most important metacharacter is the backslash, ``\``. As in Python |
| string literals, the backslash can be followed by various characters to signal |
| various special sequences. It's also used to escape all the metacharacters so |
| you can still match them in patterns; for example, if you need to match a ``[`` |
| or ``\``, you can precede them with a backslash to remove their special |
| meaning: ``\[`` or ``\\``. |
| |
| Some of the special sequences beginning with ``'\'`` represent predefined sets |
| of characters that are often useful, such as the set of digits, the set of |
| letters, or the set of anything that isn't whitespace. The following predefined |
| special sequences are a subset of those available. The equivalent classes are |
| for byte string patterns. For a complete list of sequences and expanded class |
| definitions for Unicode string patterns, see the last part of |
| :ref:`Regular Expression Syntax <re-syntax>`. |
| |
| ``\d`` |
| Matches any decimal digit; this is equivalent to the class ``[0-9]``. |
| |
| ``\D`` |
| Matches any non-digit character; this is equivalent to the class ``[^0-9]``. |
| |
| ``\s`` |
| Matches any whitespace character; this is equivalent to the class ``[ |
| \t\n\r\f\v]``. |
| |
| ``\S`` |
| Matches any non-whitespace character; this is equivalent to the class ``[^ |
| \t\n\r\f\v]``. |
| |
| ``\w`` |
| Matches any alphanumeric character; this is equivalent to the class |
| ``[a-zA-Z0-9_]``. |
| |
| ``\W`` |
| Matches any non-alphanumeric character; this is equivalent to the class |
| ``[^a-zA-Z0-9_]``. |
| |
| These sequences can be included inside a character class. For example, |
| ``[\s,.]`` is a character class that will match any whitespace character, or |
| ``','`` or ``'.'``. |
| |
| The final metacharacter in this section is ``.``. It matches anything except a |
| newline character, and there's an alternate mode (``re.DOTALL``) where it will |
| match even a newline. ``'.'`` is often used where you want to match "any |
| character". |
| |
| |
| Repeating Things |
| ---------------- |
| |
| Being able to match varying sets of characters is the first thing regular |
| expressions can do that isn't already possible with the methods available on |
| strings. However, if that was the only additional capability of regexes, they |
| wouldn't be much of an advance. Another capability is that you can specify that |
| portions of the RE must be repeated a certain number of times. |
| |
| The first metacharacter for repeating things that we'll look at is ``*``. ``*`` |
| doesn't match the literal character ``*``; instead, it specifies that the |
| previous character can be matched zero or more times, instead of exactly once. |
| |
| For example, ``ca*t`` will match ``ct`` (0 ``a`` characters), ``cat`` (1 ``a``), |
| ``caaat`` (3 ``a`` characters), and so forth. The RE engine has various |
| internal limitations stemming from the size of C's ``int`` type that will |
| prevent it from matching over 2 billion ``a`` characters; you probably don't |
| have enough memory to construct a string that large, so you shouldn't run into |
| that limit. |
| |
| Repetitions such as ``*`` are :dfn:`greedy`; when repeating a RE, the matching |
| engine will try to repeat it as many times as possible. If later portions of the |
| pattern don't match, the matching engine will then back up and try again with |
| few repetitions. |
| |
| A step-by-step example will make this more obvious. Let's consider the |
| expression ``a[bcd]*b``. This matches the letter ``'a'``, zero or more letters |
| from the class ``[bcd]``, and finally ends with a ``'b'``. Now imagine matching |
| this RE against the string ``abcbd``. |
| |
| +------+-----------+---------------------------------+ |
| | Step | Matched | Explanation | |
| +======+===========+=================================+ |
| | 1 | ``a`` | The ``a`` in the RE matches. | |
| +------+-----------+---------------------------------+ |
| | 2 | ``abcbd`` | The engine matches ``[bcd]*``, | |
| | | | going as far as it can, which | |
| | | | is to the end of the string. | |
| +------+-----------+---------------------------------+ |
| | 3 | *Failure* | The engine tries to match | |
| | | | ``b``, but the current position | |
| | | | is at the end of the string, so | |
| | | | it fails. | |
| +------+-----------+---------------------------------+ |
| | 4 | ``abcb`` | Back up, so that ``[bcd]*`` | |
| | | | matches one less character. | |
| +------+-----------+---------------------------------+ |
| | 5 | *Failure* | Try ``b`` again, but the | |
| | | | current position is at the last | |
| | | | character, which is a ``'d'``. | |
| +------+-----------+---------------------------------+ |
| | 6 | ``abc`` | Back up again, so that | |
| | | | ``[bcd]*`` is only matching | |
| | | | ``bc``. | |
| +------+-----------+---------------------------------+ |
| | 6 | ``abcb`` | Try ``b`` again. This time | |
| | | | the character at the | |
| | | | current position is ``'b'``, so | |
| | | | it succeeds. | |
| +------+-----------+---------------------------------+ |
| |
| The end of the RE has now been reached, and it has matched ``abcb``. This |
| demonstrates how the matching engine goes as far as it can at first, and if no |
| match is found it will then progressively back up and retry the rest of the RE |
| again and again. It will back up until it has tried zero matches for |
| ``[bcd]*``, and if that subsequently fails, the engine will conclude that the |
| string doesn't match the RE at all. |
| |
| Another repeating metacharacter is ``+``, which matches one or more times. Pay |
| careful attention to the difference between ``*`` and ``+``; ``*`` matches |
| *zero* or more times, so whatever's being repeated may not be present at all, |
| while ``+`` requires at least *one* occurrence. To use a similar example, |
| ``ca+t`` will match ``cat`` (1 ``a``), ``caaat`` (3 ``a``'s), but won't match |
| ``ct``. |
| |
| There are two more repeating qualifiers. The question mark character, ``?``, |
| matches either once or zero times; you can think of it as marking something as |
| being optional. For example, ``home-?brew`` matches either ``homebrew`` or |
| ``home-brew``. |
| |
| The most complicated repeated qualifier is ``{m,n}``, where *m* and *n* are |
| decimal integers. This qualifier means there must be at least *m* repetitions, |
| and at most *n*. For example, ``a/{1,3}b`` will match ``a/b``, ``a//b``, and |
| ``a///b``. It won't match ``ab``, which has no slashes, or ``a////b``, which |
| has four. |
| |
| You can omit either *m* or *n*; in that case, a reasonable value is assumed for |
| the missing value. Omitting *m* is interpreted as a lower limit of 0, while |
| omitting *n* results in an upper bound of infinity --- actually, the upper bound |
| is the 2-billion limit mentioned earlier, but that might as well be infinity. |
| |
| Readers of a reductionist bent may notice that the three other qualifiers can |
| all be expressed using this notation. ``{0,}`` is the same as ``*``, ``{1,}`` |
| is equivalent to ``+``, and ``{0,1}`` is the same as ``?``. It's better to use |
| ``*``, ``+``, or ``?`` when you can, simply because they're shorter and easier |
| to read. |
| |
| |
| Using Regular Expressions |
| ========================= |
| |
| Now that we've looked at some simple regular expressions, how do we actually use |
| them in Python? The :mod:`re` module provides an interface to the regular |
| expression engine, allowing you to compile REs into objects and then perform |
| matches with them. |
| |
| |
| Compiling Regular Expressions |
| ----------------------------- |
| |
| Regular expressions are compiled into pattern objects, which have |
| methods for various operations such as searching for pattern matches or |
| performing string substitutions. :: |
| |
| >>> import re |
| >>> p = re.compile('ab*') |
| >>> print p |
| <_sre.SRE_Pattern object at 0x...> |
| |
| :func:`re.compile` also accepts an optional *flags* argument, used to enable |
| various special features and syntax variations. We'll go over the available |
| settings later, but for now a single example will do:: |
| |
| >>> p = re.compile('ab*', re.IGNORECASE) |
| |
| The RE is passed to :func:`re.compile` as a string. REs are handled as strings |
| because regular expressions aren't part of the core Python language, and no |
| special syntax was created for expressing them. (There are applications that |
| don't need REs at all, so there's no need to bloat the language specification by |
| including them.) Instead, the :mod:`re` module is simply a C extension module |
| included with Python, just like the :mod:`socket` or :mod:`zlib` modules. |
| |
| Putting REs in strings keeps the Python language simpler, but has one |
| disadvantage which is the topic of the next section. |
| |
| |
| The Backslash Plague |
| -------------------- |
| |
| As stated earlier, regular expressions use the backslash character (``'\'``) to |
| indicate special forms or to allow special characters to be used without |
| invoking their special meaning. This conflicts with Python's usage of the same |
| character for the same purpose in string literals. |
| |
| Let's say you want to write a RE that matches the string ``\section``, which |
| might be found in a LaTeX file. To figure out what to write in the program |
| code, start with the desired string to be matched. Next, you must escape any |
| backslashes and other metacharacters by preceding them with a backslash, |
| resulting in the string ``\\section``. The resulting string that must be passed |
| to :func:`re.compile` must be ``\\section``. However, to express this as a |
| Python string literal, both backslashes must be escaped *again*. |
| |
| +-------------------+------------------------------------------+ |
| | Characters | Stage | |
| +===================+==========================================+ |
| | ``\section`` | Text string to be matched | |
| +-------------------+------------------------------------------+ |
| | ``\\section`` | Escaped backslash for :func:`re.compile` | |
| +-------------------+------------------------------------------+ |
| | ``"\\\\section"`` | Escaped backslashes for a string literal | |
| +-------------------+------------------------------------------+ |
| |
| In short, to match a literal backslash, one has to write ``'\\\\'`` as the RE |
| string, because the regular expression must be ``\\``, and each backslash must |
| be expressed as ``\\`` inside a regular Python string literal. In REs that |
| feature backslashes repeatedly, this leads to lots of repeated backslashes and |
| makes the resulting strings difficult to understand. |
| |
| The solution is to use Python's raw string notation for regular expressions; |
| backslashes are not handled in any special way in a string literal prefixed with |
| ``'r'``, so ``r"\n"`` is a two-character string containing ``'\'`` and ``'n'``, |
| while ``"\n"`` is a one-character string containing a newline. Regular |
| expressions will often be written in Python code using this raw string notation. |
| |
| +-------------------+------------------+ |
| | Regular String | Raw string | |
| +===================+==================+ |
| | ``"ab*"`` | ``r"ab*"`` | |
| +-------------------+------------------+ |
| | ``"\\\\section"`` | ``r"\\section"`` | |
| +-------------------+------------------+ |
| | ``"\\w+\\s+\\1"`` | ``r"\w+\s+\1"`` | |
| +-------------------+------------------+ |
| |
| |
| Performing Matches |
| ------------------ |
| |
| Once you have an object representing a compiled regular expression, what do you |
| do with it? Pattern objects have several methods and attributes. |
| Only the most significant ones will be covered here; consult the :mod:`re` docs |
| for a complete listing. |
| |
| +------------------+-----------------------------------------------+ |
| | Method/Attribute | Purpose | |
| +==================+===============================================+ |
| | ``match()`` | Determine if the RE matches at the beginning | |
| | | of the string. | |
| +------------------+-----------------------------------------------+ |
| | ``search()`` | Scan through a string, looking for any | |
| | | location where this RE matches. | |
| +------------------+-----------------------------------------------+ |
| | ``findall()`` | Find all substrings where the RE matches, and | |
| | | returns them as a list. | |
| +------------------+-----------------------------------------------+ |
| | ``finditer()`` | Find all substrings where the RE matches, and | |
| | | returns them as an :term:`iterator`. | |
| +------------------+-----------------------------------------------+ |
| |
| :meth:`match` and :meth:`search` return ``None`` if no match can be found. If |
| they're successful, a ``MatchObject`` instance is returned, containing |
| information about the match: where it starts and ends, the substring it matched, |
| and more. |
| |
| You can learn about this by interactively experimenting with the :mod:`re` |
| module. If you have Tkinter available, you may also want to look at |
| :file:`Tools/scripts/redemo.py`, a demonstration program included with the |
| Python distribution. It allows you to enter REs and strings, and displays |
| whether the RE matches or fails. :file:`redemo.py` can be quite useful when |
| trying to debug a complicated RE. Phil Schwartz's `Kodos |
| <http://kodos.sourceforge.net/>`_ is also an interactive tool for developing and |
| testing RE patterns. |
| |
| This HOWTO uses the standard Python interpreter for its examples. First, run the |
| Python interpreter, import the :mod:`re` module, and compile a RE:: |
| |
| Python 2.2.2 (#1, Feb 10 2003, 12:57:01) |
| >>> import re |
| >>> p = re.compile('[a-z]+') |
| >>> p |
| <_sre.SRE_Pattern object at 0x...> |
| |
| Now, you can try matching various strings against the RE ``[a-z]+``. An empty |
| string shouldn't match at all, since ``+`` means 'one or more repetitions'. |
| :meth:`match` should return ``None`` in this case, which will cause the |
| interpreter to print no output. You can explicitly print the result of |
| :meth:`match` to make this clear. :: |
| |
| >>> p.match("") |
| >>> print p.match("") |
| None |
| |
| Now, let's try it on a string that it should match, such as ``tempo``. In this |
| case, :meth:`match` will return a :class:`MatchObject`, so you should store the |
| result in a variable for later use. :: |
| |
| >>> m = p.match('tempo') |
| >>> print m |
| <_sre.SRE_Match object at 0x...> |
| |
| Now you can query the :class:`MatchObject` for information about the matching |
| string. :class:`MatchObject` instances also have several methods and |
| attributes; the most important ones are: |
| |
| +------------------+--------------------------------------------+ |
| | Method/Attribute | Purpose | |
| +==================+============================================+ |
| | ``group()`` | Return the string matched by the RE | |
| +------------------+--------------------------------------------+ |
| | ``start()`` | Return the starting position of the match | |
| +------------------+--------------------------------------------+ |
| | ``end()`` | Return the ending position of the match | |
| +------------------+--------------------------------------------+ |
| | ``span()`` | Return a tuple containing the (start, end) | |
| | | positions of the match | |
| +------------------+--------------------------------------------+ |
| |
| Trying these methods will soon clarify their meaning:: |
| |
| >>> m.group() |
| 'tempo' |
| >>> m.start(), m.end() |
| (0, 5) |
| >>> m.span() |
| (0, 5) |
| |
| :meth:`group` returns the substring that was matched by the RE. :meth:`start` |
| and :meth:`end` return the starting and ending index of the match. :meth:`span` |
| returns both start and end indexes in a single tuple. Since the :meth:`match` |
| method only checks if the RE matches at the start of a string, :meth:`start` |
| will always be zero. However, the :meth:`search` method of patterns |
| scans through the string, so the match may not start at zero in that |
| case. :: |
| |
| >>> print p.match('::: message') |
| None |
| >>> m = p.search('::: message') ; print m |
| <_sre.SRE_Match object at 0x...> |
| >>> m.group() |
| 'message' |
| >>> m.span() |
| (4, 11) |
| |
| In actual programs, the most common style is to store the :class:`MatchObject` |
| in a variable, and then check if it was ``None``. This usually looks like:: |
| |
| p = re.compile( ... ) |
| m = p.match( 'string goes here' ) |
| if m: |
| print 'Match found: ', m.group() |
| else: |
| print 'No match' |
| |
| Two pattern methods return all of the matches for a pattern. |
| :meth:`findall` returns a list of matching strings:: |
| |
| >>> p = re.compile('\d+') |
| >>> p.findall('12 drummers drumming, 11 pipers piping, 10 lords a-leaping') |
| ['12', '11', '10'] |
| |
| :meth:`findall` has to create the entire list before it can be returned as the |
| result. The :meth:`finditer` method returns a sequence of :class:`MatchObject` |
| instances as an :term:`iterator`. [#]_ :: |
| |
| >>> iterator = p.finditer('12 drummers drumming, 11 ... 10 ...') |
| >>> iterator |
| <callable-iterator object at 0x401833ac> |
| >>> for match in iterator: |
| ... print match.span() |
| ... |
| (0, 2) |
| (22, 24) |
| (29, 31) |
| |
| |
| Module-Level Functions |
| ---------------------- |
| |
| You don't have to create a pattern object and call its methods; the |
| :mod:`re` module also provides top-level functions called :func:`match`, |
| :func:`search`, :func:`findall`, :func:`sub`, and so forth. These functions |
| take the same arguments as the corresponding pattern method, with |
| the RE string added as the first argument, and still return either ``None`` or a |
| :class:`MatchObject` instance. :: |
| |
| >>> print re.match(r'From\s+', 'Fromage amk') |
| None |
| >>> re.match(r'From\s+', 'From amk Thu May 14 19:12:10 1998') |
| <_sre.SRE_Match object at 0x...> |
| |
| Under the hood, these functions simply create a pattern object for you |
| and call the appropriate method on it. They also store the compiled object in a |
| cache, so future calls using the same RE are faster. |
| |
| Should you use these module-level functions, or should you get the |
| pattern and call its methods yourself? That choice depends on how |
| frequently the RE will be used, and on your personal coding style. If the RE is |
| being used at only one point in the code, then the module functions are probably |
| more convenient. If a program contains a lot of regular expressions, or re-uses |
| the same ones in several locations, then it might be worthwhile to collect all |
| the definitions in one place, in a section of code that compiles all the REs |
| ahead of time. To take an example from the standard library, here's an extract |
| from :file:`xmllib.py`:: |
| |
| ref = re.compile( ... ) |
| entityref = re.compile( ... ) |
| charref = re.compile( ... ) |
| starttagopen = re.compile( ... ) |
| |
| I generally prefer to work with the compiled object, even for one-time uses, but |
| few people will be as much of a purist about this as I am. |
| |
| |
| Compilation Flags |
| ----------------- |
| |
| Compilation flags let you modify some aspects of how regular expressions work. |
| Flags are available in the :mod:`re` module under two names, a long name such as |
| :const:`IGNORECASE` and a short, one-letter form such as :const:`I`. (If you're |
| familiar with Perl's pattern modifiers, the one-letter forms use the same |
| letters; the short form of :const:`re.VERBOSE` is :const:`re.X`, for example.) |
| Multiple flags can be specified by bitwise OR-ing them; ``re.I | re.M`` sets |
| both the :const:`I` and :const:`M` flags, for example. |
| |
| Here's a table of the available flags, followed by a more detailed explanation |
| of each one. |
| |
| +---------------------------------+--------------------------------------------+ |
| | Flag | Meaning | |
| +=================================+============================================+ |
| | :const:`DOTALL`, :const:`S` | Make ``.`` match any character, including | |
| | | newlines | |
| +---------------------------------+--------------------------------------------+ |
| | :const:`IGNORECASE`, :const:`I` | Do case-insensitive matches | |
| +---------------------------------+--------------------------------------------+ |
| | :const:`LOCALE`, :const:`L` | Do a locale-aware match | |
| +---------------------------------+--------------------------------------------+ |
| | :const:`MULTILINE`, :const:`M` | Multi-line matching, affecting ``^`` and | |
| | | ``$`` | |
| +---------------------------------+--------------------------------------------+ |
| | :const:`VERBOSE`, :const:`X` | Enable verbose REs, which can be organized | |
| | | more cleanly and understandably. | |
| +---------------------------------+--------------------------------------------+ |
| | :const:`UNICODE`, :const:`U` | Makes several escapes like ``\w``, ``\b``, | |
| | | ``\s`` and ``\d`` dependent on the Unicode | |
| | | character database. | |
| +---------------------------------+--------------------------------------------+ |
| |
| |
| .. data:: I |
| IGNORECASE |
| :noindex: |
| |
| Perform case-insensitive matching; character class and literal strings will |
| match letters by ignoring case. For example, ``[A-Z]`` will match lowercase |
| letters, too, and ``Spam`` will match ``Spam``, ``spam``, or ``spAM``. This |
| lowercasing doesn't take the current locale into account; it will if you also |
| set the :const:`LOCALE` flag. |
| |
| |
| .. data:: L |
| LOCALE |
| :noindex: |
| |
| Make ``\w``, ``\W``, ``\b``, and ``\B``, dependent on the current locale. |
| |
| Locales are a feature of the C library intended to help in writing programs that |
| take account of language differences. For example, if you're processing French |
| text, you'd want to be able to write ``\w+`` to match words, but ``\w`` only |
| matches the character class ``[A-Za-z]``; it won't match ``'é'`` or ``'ç'``. If |
| your system is configured properly and a French locale is selected, certain C |
| functions will tell the program that ``'é'`` should also be considered a letter. |
| Setting the :const:`LOCALE` flag when compiling a regular expression will cause |
| the resulting compiled object to use these C functions for ``\w``; this is |
| slower, but also enables ``\w+`` to match French words as you'd expect. |
| |
| |
| .. data:: M |
| MULTILINE |
| :noindex: |
| |
| (``^`` and ``$`` haven't been explained yet; they'll be introduced in section |
| :ref:`more-metacharacters`.) |
| |
| Usually ``^`` matches only at the beginning of the string, and ``$`` matches |
| only at the end of the string and immediately before the newline (if any) at the |
| end of the string. When this flag is specified, ``^`` matches at the beginning |
| of the string and at the beginning of each line within the string, immediately |
| following each newline. Similarly, the ``$`` metacharacter matches either at |
| the end of the string and at the end of each line (immediately preceding each |
| newline). |
| |
| |
| .. data:: S |
| DOTALL |
| :noindex: |
| |
| Makes the ``'.'`` special character match any character at all, including a |
| newline; without this flag, ``'.'`` will match anything *except* a newline. |
| |
| |
| .. data:: U |
| UNICODE |
| :noindex: |
| |
| Make ``\w``, ``\W``, ``\b``, ``\B``, ``\d``, ``\D``, ``\s`` and ``\S`` |
| dependent on the Unicode character properties database. |
| |
| |
| .. data:: X |
| VERBOSE |
| :noindex: |
| |
| This flag allows you to write regular expressions that are more readable by |
| granting you more flexibility in how you can format them. When this flag has |
| been specified, whitespace within the RE string is ignored, except when the |
| whitespace is in a character class or preceded by an unescaped backslash; this |
| lets you organize and indent the RE more clearly. This flag also lets you put |
| comments within a RE that will be ignored by the engine; comments are marked by |
| a ``'#'`` that's neither in a character class or preceded by an unescaped |
| backslash. |
| |
| For example, here's a RE that uses :const:`re.VERBOSE`; see how much easier it |
| is to read? :: |
| |
| charref = re.compile(r""" |
| &[#] # Start of a numeric entity reference |
| ( |
| 0[0-7]+ # Octal form |
| | [0-9]+ # Decimal form |
| | x[0-9a-fA-F]+ # Hexadecimal form |
| ) |
| ; # Trailing semicolon |
| """, re.VERBOSE) |
| |
| Without the verbose setting, the RE would look like this:: |
| |
| charref = re.compile("&#(0[0-7]+" |
| "|[0-9]+" |
| "|x[0-9a-fA-F]+);") |
| |
| In the above example, Python's automatic concatenation of string literals has |
| been used to break up the RE into smaller pieces, but it's still more difficult |
| to understand than the version using :const:`re.VERBOSE`. |
| |
| |
| More Pattern Power |
| ================== |
| |
| So far we've only covered a part of the features of regular expressions. In |
| this section, we'll cover some new metacharacters, and how to use groups to |
| retrieve portions of the text that was matched. |
| |
| |
| .. _more-metacharacters: |
| |
| More Metacharacters |
| ------------------- |
| |
| There are some metacharacters that we haven't covered yet. Most of them will be |
| covered in this section. |
| |
| Some of the remaining metacharacters to be discussed are :dfn:`zero-width |
| assertions`. They don't cause the engine to advance through the string; |
| instead, they consume no characters at all, and simply succeed or fail. For |
| example, ``\b`` is an assertion that the current position is located at a word |
| boundary; the position isn't changed by the ``\b`` at all. This means that |
| zero-width assertions should never be repeated, because if they match once at a |
| given location, they can obviously be matched an infinite number of times. |
| |
| ``|`` |
| Alternation, or the "or" operator. If A and B are regular expressions, |
| ``A|B`` will match any string that matches either ``A`` or ``B``. ``|`` has very |
| low precedence in order to make it work reasonably when you're alternating |
| multi-character strings. ``Crow|Servo`` will match either ``Crow`` or ``Servo``, |
| not ``Cro``, a ``'w'`` or an ``'S'``, and ``ervo``. |
| |
| To match a literal ``'|'``, use ``\|``, or enclose it inside a character class, |
| as in ``[|]``. |
| |
| ``^`` |
| Matches at the beginning of lines. Unless the :const:`MULTILINE` flag has been |
| set, this will only match at the beginning of the string. In :const:`MULTILINE` |
| mode, this also matches immediately after each newline within the string. |
| |
| For example, if you wish to match the word ``From`` only at the beginning of a |
| line, the RE to use is ``^From``. :: |
| |
| >>> print re.search('^From', 'From Here to Eternity') |
| <_sre.SRE_Match object at 0x...> |
| >>> print re.search('^From', 'Reciting From Memory') |
| None |
| |
| .. To match a literal \character{\^}, use \regexp{\e\^} or enclose it |
| .. inside a character class, as in \regexp{[{\e}\^]}. |
| |
| ``$`` |
| Matches at the end of a line, which is defined as either the end of the string, |
| or any location followed by a newline character. :: |
| |
| >>> print re.search('}$', '{block}') |
| <_sre.SRE_Match object at 0x...> |
| >>> print re.search('}$', '{block} ') |
| None |
| >>> print re.search('}$', '{block}\n') |
| <_sre.SRE_Match object at 0x...> |
| |
| To match a literal ``'$'``, use ``\$`` or enclose it inside a character class, |
| as in ``[$]``. |
| |
| ``\A`` |
| Matches only at the start of the string. When not in :const:`MULTILINE` mode, |
| ``\A`` and ``^`` are effectively the same. In :const:`MULTILINE` mode, they're |
| different: ``\A`` still matches only at the beginning of the string, but ``^`` |
| may match at any location inside the string that follows a newline character. |
| |
| ``\Z`` |
| Matches only at the end of the string. |
| |
| ``\b`` |
| Word boundary. This is a zero-width assertion that matches only at the |
| beginning or end of a word. A word is defined as a sequence of alphanumeric |
| characters, so the end of a word is indicated by whitespace or a |
| non-alphanumeric character. |
| |
| The following example matches ``class`` only when it's a complete word; it won't |
| match when it's contained inside another word. :: |
| |
| >>> p = re.compile(r'\bclass\b') |
| >>> print p.search('no class at all') |
| <_sre.SRE_Match object at 0x...> |
| >>> print p.search('the declassified algorithm') |
| None |
| >>> print p.search('one subclass is') |
| None |
| |
| There are two subtleties you should remember when using this special sequence. |
| First, this is the worst collision between Python's string literals and regular |
| expression sequences. In Python's string literals, ``\b`` is the backspace |
| character, ASCII value 8. If you're not using raw strings, then Python will |
| convert the ``\b`` to a backspace, and your RE won't match as you expect it to. |
| The following example looks the same as our previous RE, but omits the ``'r'`` |
| in front of the RE string. :: |
| |
| >>> p = re.compile('\bclass\b') |
| >>> print p.search('no class at all') |
| None |
| >>> print p.search('\b' + 'class' + '\b') |
| <_sre.SRE_Match object at 0x...> |
| |
| Second, inside a character class, where there's no use for this assertion, |
| ``\b`` represents the backspace character, for compatibility with Python's |
| string literals. |
| |
| ``\B`` |
| Another zero-width assertion, this is the opposite of ``\b``, only matching when |
| the current position is not at a word boundary. |
| |
| |
| Grouping |
| -------- |
| |
| Frequently you need to obtain more information than just whether the RE matched |
| or not. Regular expressions are often used to dissect strings by writing a RE |
| divided into several subgroups which match different components of interest. |
| For example, an RFC-822 header line is divided into a header name and a value, |
| separated by a ``':'``, like this:: |
| |
| From: author@example.com |
| User-Agent: Thunderbird 1.5.0.9 (X11/20061227) |
| MIME-Version: 1.0 |
| To: editor@example.com |
| |
| This can be handled by writing a regular expression which matches an entire |
| header line, and has one group which matches the header name, and another group |
| which matches the header's value. |
| |
| Groups are marked by the ``'('``, ``')'`` metacharacters. ``'('`` and ``')'`` |
| have much the same meaning as they do in mathematical expressions; they group |
| together the expressions contained inside them, and you can repeat the contents |
| of a group with a repeating qualifier, such as ``*``, ``+``, ``?``, or |
| ``{m,n}``. For example, ``(ab)*`` will match zero or more repetitions of |
| ``ab``. :: |
| |
| >>> p = re.compile('(ab)*') |
| >>> print p.match('ababababab').span() |
| (0, 10) |
| |
| Groups indicated with ``'('``, ``')'`` also capture the starting and ending |
| index of the text that they match; this can be retrieved by passing an argument |
| to :meth:`group`, :meth:`start`, :meth:`end`, and :meth:`span`. Groups are |
| numbered starting with 0. Group 0 is always present; it's the whole RE, so |
| :class:`MatchObject` methods all have group 0 as their default argument. Later |
| we'll see how to express groups that don't capture the span of text that they |
| match. :: |
| |
| >>> p = re.compile('(a)b') |
| >>> m = p.match('ab') |
| >>> m.group() |
| 'ab' |
| >>> m.group(0) |
| 'ab' |
| |
| Subgroups are numbered from left to right, from 1 upward. Groups can be nested; |
| to determine the number, just count the opening parenthesis characters, going |
| from left to right. :: |
| |
| >>> p = re.compile('(a(b)c)d') |
| >>> m = p.match('abcd') |
| >>> m.group(0) |
| 'abcd' |
| >>> m.group(1) |
| 'abc' |
| >>> m.group(2) |
| 'b' |
| |
| :meth:`group` can be passed multiple group numbers at a time, in which case it |
| will return a tuple containing the corresponding values for those groups. :: |
| |
| >>> m.group(2,1,2) |
| ('b', 'abc', 'b') |
| |
| The :meth:`groups` method returns a tuple containing the strings for all the |
| subgroups, from 1 up to however many there are. :: |
| |
| >>> m.groups() |
| ('abc', 'b') |
| |
| Backreferences in a pattern allow you to specify that the contents of an earlier |
| capturing group must also be found at the current location in the string. For |
| example, ``\1`` will succeed if the exact contents of group 1 can be found at |
| the current position, and fails otherwise. Remember that Python's string |
| literals also use a backslash followed by numbers to allow including arbitrary |
| characters in a string, so be sure to use a raw string when incorporating |
| backreferences in a RE. |
| |
| For example, the following RE detects doubled words in a string. :: |
| |
| >>> p = re.compile(r'(\b\w+)\s+\1') |
| >>> p.search('Paris in the the spring').group() |
| 'the the' |
| |
| Backreferences like this aren't often useful for just searching through a string |
| --- there are few text formats which repeat data in this way --- but you'll soon |
| find out that they're *very* useful when performing string substitutions. |
| |
| |
| Non-capturing and Named Groups |
| ------------------------------ |
| |
| Elaborate REs may use many groups, both to capture substrings of interest, and |
| to group and structure the RE itself. In complex REs, it becomes difficult to |
| keep track of the group numbers. There are two features which help with this |
| problem. Both of them use a common syntax for regular expression extensions, so |
| we'll look at that first. |
| |
| Perl 5 added several additional features to standard regular expressions, and |
| the Python :mod:`re` module supports most of them. It would have been |
| difficult to choose new single-keystroke metacharacters or new special sequences |
| beginning with ``\`` to represent the new features without making Perl's regular |
| expressions confusingly different from standard REs. If you chose ``&`` as a |
| new metacharacter, for example, old expressions would be assuming that ``&`` was |
| a regular character and wouldn't have escaped it by writing ``\&`` or ``[&]``. |
| |
| The solution chosen by the Perl developers was to use ``(?...)`` as the |
| extension syntax. ``?`` immediately after a parenthesis was a syntax error |
| because the ``?`` would have nothing to repeat, so this didn't introduce any |
| compatibility problems. The characters immediately after the ``?`` indicate |
| what extension is being used, so ``(?=foo)`` is one thing (a positive lookahead |
| assertion) and ``(?:foo)`` is something else (a non-capturing group containing |
| the subexpression ``foo``). |
| |
| Python adds an extension syntax to Perl's extension syntax. If the first |
| character after the question mark is a ``P``, you know that it's an extension |
| that's specific to Python. Currently there are two such extensions: |
| ``(?P<name>...)`` defines a named group, and ``(?P=name)`` is a backreference to |
| a named group. If future versions of Perl 5 add similar features using a |
| different syntax, the :mod:`re` module will be changed to support the new |
| syntax, while preserving the Python-specific syntax for compatibility's sake. |
| |
| Now that we've looked at the general extension syntax, we can return to the |
| features that simplify working with groups in complex REs. Since groups are |
| numbered from left to right and a complex expression may use many groups, it can |
| become difficult to keep track of the correct numbering. Modifying such a |
| complex RE is annoying, too: insert a new group near the beginning and you |
| change the numbers of everything that follows it. |
| |
| Sometimes you'll want to use a group to collect a part of a regular expression, |
| but aren't interested in retrieving the group's contents. You can make this fact |
| explicit by using a non-capturing group: ``(?:...)``, where you can replace the |
| ``...`` with any other regular expression. :: |
| |
| >>> m = re.match("([abc])+", "abc") |
| >>> m.groups() |
| ('c',) |
| >>> m = re.match("(?:[abc])+", "abc") |
| >>> m.groups() |
| () |
| |
| Except for the fact that you can't retrieve the contents of what the group |
| matched, a non-capturing group behaves exactly the same as a capturing group; |
| you can put anything inside it, repeat it with a repetition metacharacter such |
| as ``*``, and nest it within other groups (capturing or non-capturing). |
| ``(?:...)`` is particularly useful when modifying an existing pattern, since you |
| can add new groups without changing how all the other groups are numbered. It |
| should be mentioned that there's no performance difference in searching between |
| capturing and non-capturing groups; neither form is any faster than the other. |
| |
| A more significant feature is named groups: instead of referring to them by |
| numbers, groups can be referenced by a name. |
| |
| The syntax for a named group is one of the Python-specific extensions: |
| ``(?P<name>...)``. *name* is, obviously, the name of the group. Named groups |
| also behave exactly like capturing groups, and additionally associate a name |
| with a group. The :class:`MatchObject` methods that deal with capturing groups |
| all accept either integers that refer to the group by number or strings that |
| contain the desired group's name. Named groups are still given numbers, so you |
| can retrieve information about a group in two ways:: |
| |
| >>> p = re.compile(r'(?P<word>\b\w+\b)') |
| >>> m = p.search( '(((( Lots of punctuation )))' ) |
| >>> m.group('word') |
| 'Lots' |
| >>> m.group(1) |
| 'Lots' |
| |
| Named groups are handy because they let you use easily-remembered names, instead |
| of having to remember numbers. Here's an example RE from the :mod:`imaplib` |
| module:: |
| |
| InternalDate = re.compile(r'INTERNALDATE "' |
| r'(?P<day>[ 123][0-9])-(?P<mon>[A-Z][a-z][a-z])-' |
| r'(?P<year>[0-9][0-9][0-9][0-9])' |
| r' (?P<hour>[0-9][0-9]):(?P<min>[0-9][0-9]):(?P<sec>[0-9][0-9])' |
| r' (?P<zonen>[-+])(?P<zoneh>[0-9][0-9])(?P<zonem>[0-9][0-9])' |
| r'"') |
| |
| It's obviously much easier to retrieve ``m.group('zonem')``, instead of having |
| to remember to retrieve group 9. |
| |
| The syntax for backreferences in an expression such as ``(...)\1`` refers to the |
| number of the group. There's naturally a variant that uses the group name |
| instead of the number. This is another Python extension: ``(?P=name)`` indicates |
| that the contents of the group called *name* should again be matched at the |
| current point. The regular expression for finding doubled words, |
| ``(\b\w+)\s+\1`` can also be written as ``(?P<word>\b\w+)\s+(?P=word)``:: |
| |
| >>> p = re.compile(r'(?P<word>\b\w+)\s+(?P=word)') |
| >>> p.search('Paris in the the spring').group() |
| 'the the' |
| |
| |
| Lookahead Assertions |
| -------------------- |
| |
| Another zero-width assertion is the lookahead assertion. Lookahead assertions |
| are available in both positive and negative form, and look like this: |
| |
| ``(?=...)`` |
| Positive lookahead assertion. This succeeds if the contained regular |
| expression, represented here by ``...``, successfully matches at the current |
| location, and fails otherwise. But, once the contained expression has been |
| tried, the matching engine doesn't advance at all; the rest of the pattern is |
| tried right where the assertion started. |
| |
| ``(?!...)`` |
| Negative lookahead assertion. This is the opposite of the positive assertion; |
| it succeeds if the contained expression *doesn't* match at the current position |
| in the string. |
| |
| To make this concrete, let's look at a case where a lookahead is useful. |
| Consider a simple pattern to match a filename and split it apart into a base |
| name and an extension, separated by a ``.``. For example, in ``news.rc``, |
| ``news`` is the base name, and ``rc`` is the filename's extension. |
| |
| The pattern to match this is quite simple: |
| |
| ``.*[.].*$`` |
| |
| Notice that the ``.`` needs to be treated specially because it's a |
| metacharacter; I've put it inside a character class. Also notice the trailing |
| ``$``; this is added to ensure that all the rest of the string must be included |
| in the extension. This regular expression matches ``foo.bar`` and |
| ``autoexec.bat`` and ``sendmail.cf`` and ``printers.conf``. |
| |
| Now, consider complicating the problem a bit; what if you want to match |
| filenames where the extension is not ``bat``? Some incorrect attempts: |
| |
| ``.*[.][^b].*$`` The first attempt above tries to exclude ``bat`` by requiring |
| that the first character of the extension is not a ``b``. This is wrong, |
| because the pattern also doesn't match ``foo.bar``. |
| |
| ``.*[.]([^b]..|.[^a].|..[^t])$`` |
| |
| The expression gets messier when you try to patch up the first solution by |
| requiring one of the following cases to match: the first character of the |
| extension isn't ``b``; the second character isn't ``a``; or the third character |
| isn't ``t``. This accepts ``foo.bar`` and rejects ``autoexec.bat``, but it |
| requires a three-letter extension and won't accept a filename with a two-letter |
| extension such as ``sendmail.cf``. We'll complicate the pattern again in an |
| effort to fix it. |
| |
| ``.*[.]([^b].?.?|.[^a]?.?|..?[^t]?)$`` |
| |
| In the third attempt, the second and third letters are all made optional in |
| order to allow matching extensions shorter than three characters, such as |
| ``sendmail.cf``. |
| |
| The pattern's getting really complicated now, which makes it hard to read and |
| understand. Worse, if the problem changes and you want to exclude both ``bat`` |
| and ``exe`` as extensions, the pattern would get even more complicated and |
| confusing. |
| |
| A negative lookahead cuts through all this confusion: |
| |
| ``.*[.](?!bat$).*$`` The negative lookahead means: if the expression ``bat`` |
| doesn't match at this point, try the rest of the pattern; if ``bat$`` does |
| match, the whole pattern will fail. The trailing ``$`` is required to ensure |
| that something like ``sample.batch``, where the extension only starts with |
| ``bat``, will be allowed. |
| |
| Excluding another filename extension is now easy; simply add it as an |
| alternative inside the assertion. The following pattern excludes filenames that |
| end in either ``bat`` or ``exe``: |
| |
| ``.*[.](?!bat$|exe$).*$`` |
| |
| |
| Modifying Strings |
| ================= |
| |
| Up to this point, we've simply performed searches against a static string. |
| Regular expressions are also commonly used to modify strings in various ways, |
| using the following pattern methods: |
| |
| +------------------+-----------------------------------------------+ |
| | Method/Attribute | Purpose | |
| +==================+===============================================+ |
| | ``split()`` | Split the string into a list, splitting it | |
| | | wherever the RE matches | |
| +------------------+-----------------------------------------------+ |
| | ``sub()`` | Find all substrings where the RE matches, and | |
| | | replace them with a different string | |
| +------------------+-----------------------------------------------+ |
| | ``subn()`` | Does the same thing as :meth:`sub`, but | |
| | | returns the new string and the number of | |
| | | replacements | |
| +------------------+-----------------------------------------------+ |
| |
| |
| Splitting Strings |
| ----------------- |
| |
| The :meth:`split` method of a pattern splits a string apart |
| wherever the RE matches, returning a list of the pieces. It's similar to the |
| :meth:`split` method of strings but provides much more generality in the |
| delimiters that you can split by; :meth:`split` only supports splitting by |
| whitespace or by a fixed string. As you'd expect, there's a module-level |
| :func:`re.split` function, too. |
| |
| |
| .. method:: .split(string [, maxsplit=0]) |
| :noindex: |
| |
| Split *string* by the matches of the regular expression. If capturing |
| parentheses are used in the RE, then their contents will also be returned as |
| part of the resulting list. If *maxsplit* is nonzero, at most *maxsplit* splits |
| are performed. |
| |
| You can limit the number of splits made, by passing a value for *maxsplit*. |
| When *maxsplit* is nonzero, at most *maxsplit* splits will be made, and the |
| remainder of the string is returned as the final element of the list. In the |
| following example, the delimiter is any sequence of non-alphanumeric characters. |
| :: |
| |
| >>> p = re.compile(r'\W+') |
| >>> p.split('This is a test, short and sweet, of split().') |
| ['This', 'is', 'a', 'test', 'short', 'and', 'sweet', 'of', 'split', ''] |
| >>> p.split('This is a test, short and sweet, of split().', 3) |
| ['This', 'is', 'a', 'test, short and sweet, of split().'] |
| |
| Sometimes you're not only interested in what the text between delimiters is, but |
| also need to know what the delimiter was. If capturing parentheses are used in |
| the RE, then their values are also returned as part of the list. Compare the |
| following calls:: |
| |
| >>> p = re.compile(r'\W+') |
| >>> p2 = re.compile(r'(\W+)') |
| >>> p.split('This... is a test.') |
| ['This', 'is', 'a', 'test', ''] |
| >>> p2.split('This... is a test.') |
| ['This', '... ', 'is', ' ', 'a', ' ', 'test', '.', ''] |
| |
| The module-level function :func:`re.split` adds the RE to be used as the first |
| argument, but is otherwise the same. :: |
| |
| >>> re.split('[\W]+', 'Words, words, words.') |
| ['Words', 'words', 'words', ''] |
| >>> re.split('([\W]+)', 'Words, words, words.') |
| ['Words', ', ', 'words', ', ', 'words', '.', ''] |
| >>> re.split('[\W]+', 'Words, words, words.', 1) |
| ['Words', 'words, words.'] |
| |
| |
| Search and Replace |
| ------------------ |
| |
| Another common task is to find all the matches for a pattern, and replace them |
| with a different string. The :meth:`sub` method takes a replacement value, |
| which can be either a string or a function, and the string to be processed. |
| |
| |
| .. method:: .sub(replacement, string[, count=0]) |
| :noindex: |
| |
| Returns the string obtained by replacing the leftmost non-overlapping |
| occurrences of the RE in *string* by the replacement *replacement*. If the |
| pattern isn't found, *string* is returned unchanged. |
| |
| The optional argument *count* is the maximum number of pattern occurrences to be |
| replaced; *count* must be a non-negative integer. The default value of 0 means |
| to replace all occurrences. |
| |
| Here's a simple example of using the :meth:`sub` method. It replaces colour |
| names with the word ``colour``:: |
| |
| >>> p = re.compile( '(blue|white|red)') |
| >>> p.sub( 'colour', 'blue socks and red shoes') |
| 'colour socks and colour shoes' |
| >>> p.sub( 'colour', 'blue socks and red shoes', count=1) |
| 'colour socks and red shoes' |
| |
| The :meth:`subn` method does the same work, but returns a 2-tuple containing the |
| new string value and the number of replacements that were performed:: |
| |
| >>> p = re.compile( '(blue|white|red)') |
| >>> p.subn( 'colour', 'blue socks and red shoes') |
| ('colour socks and colour shoes', 2) |
| >>> p.subn( 'colour', 'no colours at all') |
| ('no colours at all', 0) |
| |
| Empty matches are replaced only when they're not adjacent to a previous match. |
| :: |
| |
| >>> p = re.compile('x*') |
| >>> p.sub('-', 'abxd') |
| '-a-b-d-' |
| |
| If *replacement* is a string, any backslash escapes in it are processed. That |
| is, ``\n`` is converted to a single newline character, ``\r`` is converted to a |
| carriage return, and so forth. Unknown escapes such as ``\j`` are left alone. |
| Backreferences, such as ``\6``, are replaced with the substring matched by the |
| corresponding group in the RE. This lets you incorporate portions of the |
| original text in the resulting replacement string. |
| |
| This example matches the word ``section`` followed by a string enclosed in |
| ``{``, ``}``, and changes ``section`` to ``subsection``:: |
| |
| >>> p = re.compile('section{ ( [^}]* ) }', re.VERBOSE) |
| >>> p.sub(r'subsection{\1}','section{First} section{second}') |
| 'subsection{First} subsection{second}' |
| |
| There's also a syntax for referring to named groups as defined by the |
| ``(?P<name>...)`` syntax. ``\g<name>`` will use the substring matched by the |
| group named ``name``, and ``\g<number>`` uses the corresponding group number. |
| ``\g<2>`` is therefore equivalent to ``\2``, but isn't ambiguous in a |
| replacement string such as ``\g<2>0``. (``\20`` would be interpreted as a |
| reference to group 20, not a reference to group 2 followed by the literal |
| character ``'0'``.) The following substitutions are all equivalent, but use all |
| three variations of the replacement string. :: |
| |
| >>> p = re.compile('section{ (?P<name> [^}]* ) }', re.VERBOSE) |
| >>> p.sub(r'subsection{\1}','section{First}') |
| 'subsection{First}' |
| >>> p.sub(r'subsection{\g<1>}','section{First}') |
| 'subsection{First}' |
| >>> p.sub(r'subsection{\g<name>}','section{First}') |
| 'subsection{First}' |
| |
| *replacement* can also be a function, which gives you even more control. If |
| *replacement* is a function, the function is called for every non-overlapping |
| occurrence of *pattern*. On each call, the function is passed a |
| :class:`MatchObject` argument for the match and can use this information to |
| compute the desired replacement string and return it. |
| |
| In the following example, the replacement function translates decimals into |
| hexadecimal:: |
| |
| >>> def hexrepl( match ): |
| ... "Return the hex string for a decimal number" |
| ... value = int( match.group() ) |
| ... return hex(value) |
| ... |
| >>> p = re.compile(r'\d+') |
| >>> p.sub(hexrepl, 'Call 65490 for printing, 49152 for user code.') |
| 'Call 0xffd2 for printing, 0xc000 for user code.' |
| |
| When using the module-level :func:`re.sub` function, the pattern is passed as |
| the first argument. The pattern may be provided as an object or as a string; if |
| you need to specify regular expression flags, you must either use a |
| pattern object as the first parameter, or use embedded modifiers in the |
| pattern string, e.g. ``sub("(?i)b+", "x", "bbbb BBBB")`` returns ``'x x'``. |
| |
| |
| Common Problems |
| =============== |
| |
| Regular expressions are a powerful tool for some applications, but in some ways |
| their behaviour isn't intuitive and at times they don't behave the way you may |
| expect them to. This section will point out some of the most common pitfalls. |
| |
| |
| Use String Methods |
| ------------------ |
| |
| Sometimes using the :mod:`re` module is a mistake. If you're matching a fixed |
| string, or a single character class, and you're not using any :mod:`re` features |
| such as the :const:`IGNORECASE` flag, then the full power of regular expressions |
| may not be required. Strings have several methods for performing operations with |
| fixed strings and they're usually much faster, because the implementation is a |
| single small C loop that's been optimized for the purpose, instead of the large, |
| more generalized regular expression engine. |
| |
| One example might be replacing a single fixed string with another one; for |
| example, you might replace ``word`` with ``deed``. ``re.sub()`` seems like the |
| function to use for this, but consider the :meth:`replace` method. Note that |
| :func:`replace` will also replace ``word`` inside words, turning ``swordfish`` |
| into ``sdeedfish``, but the naive RE ``word`` would have done that, too. (To |
| avoid performing the substitution on parts of words, the pattern would have to |
| be ``\bword\b``, in order to require that ``word`` have a word boundary on |
| either side. This takes the job beyond :meth:`replace`'s abilities.) |
| |
| Another common task is deleting every occurrence of a single character from a |
| string or replacing it with another single character. You might do this with |
| something like ``re.sub('\n', ' ', S)``, but :meth:`translate` is capable of |
| doing both tasks and will be faster than any regular expression operation can |
| be. |
| |
| In short, before turning to the :mod:`re` module, consider whether your problem |
| can be solved with a faster and simpler string method. |
| |
| |
| match() versus search() |
| ----------------------- |
| |
| The :func:`match` function only checks if the RE matches at the beginning of the |
| string while :func:`search` will scan forward through the string for a match. |
| It's important to keep this distinction in mind. Remember, :func:`match` will |
| only report a successful match which will start at 0; if the match wouldn't |
| start at zero, :func:`match` will *not* report it. :: |
| |
| >>> print re.match('super', 'superstition').span() |
| (0, 5) |
| >>> print re.match('super', 'insuperable') |
| None |
| |
| On the other hand, :func:`search` will scan forward through the string, |
| reporting the first match it finds. :: |
| |
| >>> print re.search('super', 'superstition').span() |
| (0, 5) |
| >>> print re.search('super', 'insuperable').span() |
| (2, 7) |
| |
| Sometimes you'll be tempted to keep using :func:`re.match`, and just add ``.*`` |
| to the front of your RE. Resist this temptation and use :func:`re.search` |
| instead. The regular expression compiler does some analysis of REs in order to |
| speed up the process of looking for a match. One such analysis figures out what |
| the first character of a match must be; for example, a pattern starting with |
| ``Crow`` must match starting with a ``'C'``. The analysis lets the engine |
| quickly scan through the string looking for the starting character, only trying |
| the full match if a ``'C'`` is found. |
| |
| Adding ``.*`` defeats this optimization, requiring scanning to the end of the |
| string and then backtracking to find a match for the rest of the RE. Use |
| :func:`re.search` instead. |
| |
| |
| Greedy versus Non-Greedy |
| ------------------------ |
| |
| When repeating a regular expression, as in ``a*``, the resulting action is to |
| consume as much of the pattern as possible. This fact often bites you when |
| you're trying to match a pair of balanced delimiters, such as the angle brackets |
| surrounding an HTML tag. The naive pattern for matching a single HTML tag |
| doesn't work because of the greedy nature of ``.*``. :: |
| |
| >>> s = '<html><head><title>Title</title>' |
| >>> len(s) |
| 32 |
| >>> print re.match('<.*>', s).span() |
| (0, 32) |
| >>> print re.match('<.*>', s).group() |
| <html><head><title>Title</title> |
| |
| The RE matches the ``'<'`` in ``<html>``, and the ``.*`` consumes the rest of |
| the string. There's still more left in the RE, though, and the ``>`` can't |
| match at the end of the string, so the regular expression engine has to |
| backtrack character by character until it finds a match for the ``>``. The |
| final match extends from the ``'<'`` in ``<html>`` to the ``'>'`` in |
| ``</title>``, which isn't what you want. |
| |
| In this case, the solution is to use the non-greedy qualifiers ``*?``, ``+?``, |
| ``??``, or ``{m,n}?``, which match as *little* text as possible. In the above |
| example, the ``'>'`` is tried immediately after the first ``'<'`` matches, and |
| when it fails, the engine advances a character at a time, retrying the ``'>'`` |
| at every step. This produces just the right result:: |
| |
| >>> print re.match('<.*?>', s).group() |
| <html> |
| |
| (Note that parsing HTML or XML with regular expressions is painful. |
| Quick-and-dirty patterns will handle common cases, but HTML and XML have special |
| cases that will break the obvious regular expression; by the time you've written |
| a regular expression that handles all of the possible cases, the patterns will |
| be *very* complicated. Use an HTML or XML parser module for such tasks.) |
| |
| |
| Using re.VERBOSE |
| -------------------- |
| |
| By now you've probably noticed that regular expressions are a very compact |
| notation, but they're not terribly readable. REs of moderate complexity can |
| become lengthy collections of backslashes, parentheses, and metacharacters, |
| making them difficult to read and understand. |
| |
| For such REs, specifying the ``re.VERBOSE`` flag when compiling the regular |
| expression can be helpful, because it allows you to format the regular |
| expression more clearly. |
| |
| The ``re.VERBOSE`` flag has several effects. Whitespace in the regular |
| expression that *isn't* inside a character class is ignored. This means that an |
| expression such as ``dog | cat`` is equivalent to the less readable ``dog|cat``, |
| but ``[a b]`` will still match the characters ``'a'``, ``'b'``, or a space. In |
| addition, you can also put comments inside a RE; comments extend from a ``#`` |
| character to the next newline. When used with triple-quoted strings, this |
| enables REs to be formatted more neatly:: |
| |
| pat = re.compile(r""" |
| \s* # Skip leading whitespace |
| (?P<header>[^:]+) # Header name |
| \s* : # Whitespace, and a colon |
| (?P<value>.*?) # The header's value -- *? used to |
| # lose the following trailing whitespace |
| \s*$ # Trailing whitespace to end-of-line |
| """, re.VERBOSE) |
| |
| This is far more readable than:: |
| |
| pat = re.compile(r"\s*(?P<header>[^:]+)\s*:(?P<value>.*?)\s*$") |
| |
| |
| Feedback |
| ======== |
| |
| Regular expressions are a complicated topic. Did this document help you |
| understand them? Were there parts that were unclear, or Problems you |
| encountered that weren't covered here? If so, please send suggestions for |
| improvements to the author. |
| |
| The most complete book on regular expressions is almost certainly Jeffrey |
| Friedl's Mastering Regular Expressions, published by O'Reilly. Unfortunately, |
| it exclusively concentrates on Perl and Java's flavours of regular expressions, |
| and doesn't contain any Python material at all, so it won't be useful as a |
| reference for programming in Python. (The first edition covered Python's |
| now-removed :mod:`regex` module, which won't help you much.) Consider checking |
| it out from your library. |
| |
| |
| .. rubric:: Footnotes |
| |
| .. [#] Introduced in Python 2.2.2. |
| |